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SN74AHC08: SN74AHC08 and SN74AHCT08 difference

Philip
Expert 1510 points
Part Number: SN74AHC08
Other Parts Discussed in Thread: SN74AHCT08,

Hi there,

I checked both SN74AHC08 and SN74AHCT08 parts from TI site.

The main differences are on the supply and input type (CMOS and TTL).

Currently, SN74AHC08 is mounted on the board.

But if I replaced it with SN74AHCT08 part, what are the potential issue or design consideration I have to take note of? 

Please help to advise.

Best Regards,

Phil

  • 0
    Expert 1510 points

    Sorry, I made a mistake on the P/N.

    The part used/mounted is SN74AHCT08.

    But if I replaced it with SN74AHC08 part, what are the potential issue or design consideration I have to take note of?

    Kindly help to advise, thanks.

  • 0 in reply to Philip
    Guru 74080 points

    The main differences between HC and HCT are the operating voltage range and the input threshold levels. HCT parts are used in 5V systems as their supply voltage is 4.5V to 5.5V, whereas HC parts have a supply from typically 2.0V to 6.0V. The HC parts have higher input threshold voltages compared to the HCT parts. This makes HCT parts usable for level conversions between 3.3V systems and 5V systems. For example a MSP430 running at 3.3V that is connected via a 74HCT595 shift register to a 5V driven HD44780 display.

    The A in the part number is for "Advanced" which means you have a high speed device.

    By the way: Both of your part numbes link to the SN74AHCT08.

    Dennis

  • 0 in reply to Dennis Eichmann
    Expert 1510 points

    Hi Dennis,


    Thanks for your reply.

    Currently the board is using/mounted with SN74AHCT08.

    If I mount with SN74AHC08, would there be any potential design issues which I need to take care of?

    Philip

  • 0 in reply to Philip
    Guru 74080 points

    It depends on your circuit. For the supply voltage it is OK to replace the HCT with the HC type, but the input threshold level specification might not be met. If the device is powered with 5V, then you will need about 3.85V minimum as input level for high. If all of your logic was 5V before, then you can replace it. The switching characteristics are almost identical for 5V operation.

  • 0 in reply to Dennis Eichmann
    Expert 1510 points

    Sorry to ask further.

    For below two cases, please help to advise.

    1) If actual condition was 3.3V system, instead of using SN74AHC08NSR in actual application, we used SN74AHCT08NSR.

        It is out of spec of supply voltage.

       What will happen?

       Can we detect failure mode by function test? Any recommended ways to detect the failure mode?

     

    2) In another case, if our circuit is 5V system, instead of using SN74AHCT08NSR, we used up SN74AHC08NSR.

        It is within spec of supply voltage.

        What will happen?

        Since supply voltage is within spec, can we use it without any problem?

    Thanks.

  • 0 in reply to Philip
    Guru 242980 points
    When you run a device with an out-of-spec supply voltage, it might work, or it might not work, or it might blow up. In that situation, there is no specified behaviour, so it is not possible to test for deviance from the specified behaviour.

    The SN74AHC08 is guaranteed to work at 5 V, but it has different input threshold levels. Whether it works in your circuit depends on what the levels of its input signals are. (The only reason to use the SN74AHCT08 is to get the TTL thresholds, so it's likely that the replacement will not work.)